There are 18 Gorgon Eyes to obtain throughout the game. Every time you collect 6, you will gain a permanent health upgrade. Gorgon eyes are found in chests that do not glow with any color. These chests are almost always hidden out of the way or in secret areas. Though you require only 18, there are many more than 18 Gorgon Eye chests in the game. These chests will contain red orbs if you've already collected all the Eyes.

I highly recommend this chest guide for the Gorgon Eyes, Phoenix Feathers, and Muse Keys. It's detailed enough that you shouldn't need pictures or a video. Print off a copy or play with the page open on a tablet or laptop. Special thanks to keltin_2002 for their fantastic work putting the guide together.